1. Field of the Invention
This invention relates to a turning bar for contactless guidance of a tubular film web moved at a loop angle around the turning bar, including a support tube with a hollow interior which can be connected with a compressed air source, wherein the support tube has a plurality of air outlet openings which communicate with the interior and are spaced apart from each other, substantially over the entire width of the support tube within an angular area of the support tube corresponding to the loop angle of the tubular film web. An air-permeable microporous layer is applied to the exterior of the support tube and forms the surface of the latter, through which the compressed air exiting the air outlet openings of the support tube can flow, wherein the compressed air exits the microporous layer as a laminar air flow, which forms a laminar air cushion between the turning bar and the tubular film web.
2. Discussion of Related Art
The extrusion of tubular film webs, made of plastic, with tubular film extrusion installations is widely known. In the known processes, thickness tolerances inevitably occur in the produced tubular film web and these tolerances are added together on the tubular film web wound into a coil and cause problems in the continued processing steps, and this addition is countered by suitable countermeasures.
Such a measure, for example known from German Patent Reference DE 21 56 079 C2, provides a so-called turning bar draw-off as the draw-off device of the tubular film extrusion installation, which has at least one turning bar which is guided, for example in a reversible manner, on a plane extending at right angles to the extrusion direction, and which continuously distributes the position of the individual thickness irregularities in the extruded tubular film web over the entire width of the latter.
Because during a turning bar draw-off the tubular film web is necessarily guided obliquely, for example differing from an angle of 90° with respect to the longitudinal axis of the turning bar, over the surface of the latter because of the reversing movement of the draw-off, the turning bars of the known draw-off device are seated, fixed in place, not rotatingly seated, in the draw-off device, which causes considerable friction between the tubular film web conducted over the surface and the fixed turning bar.
Thus, various attempts have been made to reduce this friction between the turning bar and the tubular film web conducted over it.
A turning bar is known, for example from German Patent Reference DE 199 12 209 A1 and U.S. Pat. No. 6,398,534, wherein the interior of the turning bar can be connected to a compressed air source, and air outlet openings are provided in the surface at least in the area facing the tubular film web, which communicate with the interior. The air masses exiting the air outlet openings cause an air cushion between the tubular film web moved over the surface and the turning bar, so that ideally the tubular film web slides contactless over the surface and thus the otherwise inevitably created frictional forces are eliminated.
However, it is a problem in connection with the air-charged turning bars employed up to now that the air flows must exit from the air outlet openings at a high pressure and high air speed in order to produce an even air cushion over the entire surface of the turning bar. Because areas of the surface of the turning bar between the individual air outlet openings must necessarily remain free of holes in order to assure its stability, it is thus necessary to provide the areas free of openings with air from the adjoining air outlet openings in order to achieve as evenly as possible an air cushion. In actual use, this problem could not be satisfactory solved up to now, instead it was not possible up to now to avoid scattered contact between the tubular film web and the surface of the turning bar. Thus the production of tubular film webs with a high adhesion force by the turning bar draw-off devices has not been possible so far.
An expander device is also known from European Patent Reference EP 1 144 292 B1, by which a stretching of the web of material in the transverse direction is caused with a knobbed cylinder. For the floating guidance of the web of material, the cylinder has a gas-permeable wall made of a metal-containing material with open pores, which has a mean diameter of less than 10 μm, and which is charged with a gaseous fluid.
U.S. Pat. No. 5,423,468 describes a reversing roller for imprinted substrates, onto which an air-permeable sleeve of a porous plastic material is pushed in order to create an even air cushion.